+ // Old GCC versions get their type lattice for pointers-to-members
+ // tangled up to a degree where we need to typecast _both_ covariant
+ // types in order to be able to compare them. The other comparisons
+ // are for static member functions and thus are ordinary function
+ // pointers which work without those contortions.
+ if (static_cast<SCM (Super::*)()>(&Super::mark_smob) !=
+ static_cast<SCM (Super::*)()>(&Smob_base<Super>::mark_smob))